In recent years, a quadrature modulator such as that shown in FIG. 8 has generally been used as a modulator for digital communication. A quadrature modulator 1 shown in FIG. 8 includes multipliers 2 and 3, a 90-degree phase shifter 4, and an adder 5.
The multipliers 2 and 3 respectively receive two signals called an I-phase component (in-phase component) and a Q-phase component (quadrature component), respectively. The multiplier 2 multiplies the I-phase component by a carrier wave signal Sc from a local oscillator 6. The multiplier 3 multiplies the Q-phase component by the carrier wave signal Sc which is shifted by 90 degrees by the 90-degree phase shifter 4. The adder 5 adds the output signals from the multipliers 2 and 3 and outputs a quadrature-modulated signal S.
In the quadrature modulator 1, it has been known that the I-phase component and the Q-phase component included in the quadrature-modulated signal S to be output are not exactly orthogonal to each other due to, for example, incorrect settings or errors in the 90-degree phase shifter 4, which results in a modulation error. The modulation error is called an orthogonality error. For example, Patent Document 1 discloses a quadrature modulation device as a device for detecting orthogonality errors.
The quadrature modulation device disclosed in Patent Document 1 includes a pair of variable voltage supplies which generate a pair of DC voltages to be added to the I-phase component and the Q-phase component input to a quadrature modulator and DC voltage combination search means which adds the pair of DC voltages generated by the pair of variable voltage supplies to the I-phase component and the Q-phase component while changing the pair of DC voltages and searches for a plurality of pairs of DC voltage combinations causing the level of the modulation signal to be a reference level. According to this configuration, the quadrature modulation device disclosed in Patent Document 1 can calculate the orthogonality error of the quadrature-modulated signal on the basis of each value of the plurality of pairs of DC voltage combinations that were searched for and correct the orthogonality error.